THE LANGUAGE OF SUPERCONDUCTOR PHYSICS

The theoretical understanding of the phenomena of
superconductivity is extremely involved. It is far beyond the scope of this
guide to attempt to delve into that subject. However, in this short section, we
have emphasized some of the fundamental terms and phenomena that will make it
possible for you to conduct the experiments suggested with our Kits.

Superconductors have the ability to carry an electrical
current without loss of energy. Unlike normal conductors of electricity in which
the current is carried by individual electrons, in superconductors the current
is carried by pairs of electrons called Cooper Pairs, in honor of one
of the formulators of the famous `BCS' theory of superconductivity. When the
electrons move through a solid in Cooper Pairs, they are impervious to the
energy absorbing interactions that normal electrons suffer. To form Cooper
Pairs, a superconductor must operate below a certain temperature called the
Critical Temperature, or Tc. Superconductors made from
different materials have different values of Tc. For the new ceramic
superconductors in these Kits, Tc is about 90 Kelvin for
YBa2Cu3O7, and up to 110 Kelvin for Bi2Sr2Ca2Cu3O10. The
Critical Temperature
Kit, Complete
Exploration Kit, Super Exploration Kit,
and the new Magnetic
Susceptibility Kit are designed to allow you to measure Tc in a
simple and elegant manner.

It is not yet clear that these ceramic superconductors
indeed do conduct electricity by means of Cooper Pairs as described by the `BCS'
theory. In fact another theory called the `Resonant Valence Bond'
theory has been advanced as being more effective. This theory may explain the
gradual onset of superconductivity at a temperature around Tc in the
ceramic materials.

Since there is no loss in electrical energy when
superconductors carry an electrical current, relatively narrow wires made of
superconducting material can be used to carry huge currents. However, there is a
certain maximum current that these materials can be made to carry, above which
they stop being superconductors. This maximum current flux is referred to as the
Critical Current Density, or Jc. There has been a great deal
of effort to increase the value of Jc in the new ceramic
superconductors. For routine electrical measurements on the samples provided in
these Kits, you must remember to use electrical currents that result in current
densities that are smaller than Jc.

It has long been known that an electrical current in a
wire creates a magnetic field around the wire. The strength of the magnetic
field increases as the current in the wire is increased. Thus, on account of
their ability to carry large electrical currents without loss of energy,
superconductors are especially suited for making powerful electromagnets.
Furthermore, if the electrical current travels only through a superconductor
without having to pass through a normal conductor, then it will persist forever
resulting in the formation of a powerful permanent electromagnet (see the Superconducting Energy
Storage Kit). These permanent currents in a superconductor are referred to
as persistent currents. The magnetic field generated by the superconductor in
turn however, affects the ability of the superconductor to carry electrical
currents. In fact, as the magnetic field increases, the values of both
Tc and Jc decrease. When the magnetic field is greater
than a certain amount, the superconductor is quenched, and can carry no
superconducting current. This maximum magnetic field is called the maximum
Critical Field, or Hc. Again, this is a large field, and
even the powerful rare earth alloy magnets we will be using in our experiments
will not significantly affect our superconductors. The Complete Exploration
and Super
Exploration Kits can be used to determine both Hc and
Jc.

The experiments in these Kits delve into some of the
basic physics of superconductivity. These phenomena are explained in greater
detail in the Experiment Guide.